In the case of protective switchgear, such as power switches, motor protection switches or automatic cut-outs, and in the case of automatic circuit-breakers in general, it is desirable to sense overload currents, in particular short-circuit currents, quickly and to limit the overload currents to minimum possible values and finally to disconnect the overload currents. In the case of substantially mechanical automatic circuit-breakers, for example automatic cut-outs, use is made of so-called instantaneous trips, in which both the magnetic circuit is optimally designed and a magnet armature, often a plunger armature, quickly and forcibly makes impact with the contact system. Nevertheless, it has not been possible in practice to achieve contact opening times any shorter than about one millisecond. In this case, the short-circuit current continues to increase unhindered until contact opening. Only after contact opening is an arc produced, quickly conducted into an arc chamber and cooled at quenching plates, splitting the arc. The high arc voltages which this generates have the effect of limiting the short-circuit current and finally disconnecting it.
In the case of known automatic circuit-breakers, with a prospective short-circuit current of 6 kA, cos phi=0.6 and psi=60.degree., it is scarcely possible to obtain current time values any less than 4 ms for the rise time to the peak value, with a switching current of 4000 A, and an integral of the square of the current over time of 30,000 A.sup.2 s.
It has been suggested to use semiconductors for the current limitation in protective switchgear. This approach, however, is hampered or prevented in practice by various circumstances: 1) semiconductor elements generally have an inad equate current-limiting effect and an insufficient permissible energy absorption; 2) semiconductor elements generally have, in normal operation, a forward resistance of above 10 milliohms at 16 A; and 3) semiconductor elements also generally have an insufficient dielectric strength.
PCT International Application WO 93/11608 describes a power switch which acts as a current limiter for limiting overload currents by means of a semiconductor element with at least one controllable semiconductor with an electron source (source), an electron acceptor (drain) and a control electrode (gate) controlling the electron flow, which current limiter has characteristic curves of a field-effect transistor (FET), the load current flowing through the semiconductor element and, in the case of alternating voltage, two FETs being antiserially connected. In this case, an external control voltage is provided.